Message Waiting Indication (MWI) service is a widely used supplementary service. When there is an unheard voice message in a user's voice mailbox, the voice mailbox will prompt the user for a new voice message. After the user has heard the new voice message from the voice mailbox, the prompt will disappear automatically. The prompt is usually provided by either of the following two means:
Go off hook to listen for cue tone: when the user goes off the hook, he/she will receive a special dial tone or a voice prompt, to inform him/her there is a new voice message.
Display on telephone interface: the Calling Tapping at Flash indicator on the user's telephone flashes or the new voice message is displayed on the screen of the telephone (which requires the support of the telephone).
MWI service is also called Message Waiting Notification (MWN) service in some cases; in this document, it is referred to as Calling Tapping at Flash service collectively.
As packet switching technologies become matured increasingly, legacy CS-based telecommunication networks are evolving towards PS-based broadband telecommunication networks. In a packet voice network, Calling Tapping at Flash service is also a fundamental supplementary service, as specified in ETSI's Draft ETSI TS <03024>V<0.0.6> (2005-03) “Telecommunications and Internet Converged Services and Protocols for Advanced Networking (TISPAN); NGN Signaling Control Protocol; Message Waiting Indication (MWI) PSTN/ISDN simulation services”. The target network of the Draft is the packet core network with Session Initiation Protocol (SIP) as the call control signaling, for which ETSI is establishing the standard.
In the current version of the Draft, a method for implementing Calling Tapping at Flash service with SIP SUBSCRIBE/NOTIFY message mechanism is provided: the SIP terminal initiates a voice message subscription to the Voice Mail Application Server with an SIP SUBSCRIBE message; once there is a new voice message in the voice mailbox of the SIP terminal user, the Voice Mail Application Server notifies the SIP terminal with an SIP NOTIFY message.
In the Draft, the above method of implementing Calling Tapping at Flash service is limited to the following application case: the user terminal is an SIP terminal that can initiate a subscription of voice message event packets, while the voice mailbox is an SIP Application Server in PS domain.
However, for a practically operative packet voice telecommunication network, legacy terminals such as POTS terminals and ISDN terminals can also access the packet core network with SIP as the call control signaling by a certain access means and become registered users of the packet core network. It is evident that those terminals are unable to initiate subscriptions of voice message event packets but have to employ a network device to initiate subscriptions of voice message event packets to the Voice Mail Application Server on behalf of them.
For example, the supplementary service control unit for handling legacy supplementary service of a Plain Ordinary Telephone Service (POTS) terminal user has to initiate a subscription of voice message event packet to the Voice Mail Application Server for the POTS terminal user who has activated the Voice Mail service or activated the Calling Tapping at Flash service, with the subscriber as the POTS terminal user identity and the routing address of the supplementary service control unit included in the signaling path of the SUBSCRIBE message; after the supplementary service control unit receives a NOTIFY message containing a voice message event packet from the Voice Mail Application Server, it has to treat the voice message appropriately in accordance with the prompt means of user (either of the means mentioned above).
On the other hand, if the operator possesses both legacy CS domain and PS domain, the operator may configure the voice mailboxes of users accessing legacy CS domain in the PS domain after a new Voice Mail Application Server is provided for the PS domain, in consideration of maintenance cost. In that case, it is obvious that the above SUBSCRIBE/NOTIFY solution for voice message event packets can not be implemented, because there is not any network device in the legacy CS domain which can initiate a subscription of voice message event packet to the Voice Mail Application Server on behalf of the users in the legacy CS domain.
Actually, even for a new packet voice telecommunication network of the operator, the SIP terminal users accessing the new network are generally switched from the legacy CS domain. However, the Voice Mail System in legacy CS domain does not need voice message subscription; instead, once there is a new voice message for a subscribed user, it transfers the new voice message for the subscribed user in an ISDN User Part (ISUP) message or a SMS message; whereas, after the user is switched from the legacy CS domain to the PS domain and uses an SIP terminal, he/she has to initiate a subscription request from the SIP terminal to obtain new voice messages.
On the other hand, when the operator constructs a new packet voice telecommunication network, the operator usually needs to utilize the Voice Mail System in the legacy CS domain during the transition stage; that is to say, the voice mailboxes of PS users are located in the legacy CS domain. At that time, if a PS user intends to subscribe voice message event packets, the SUBSCRIBE message for the subscription will be terminated by the Media Gateway Control Unit (MGCU) between the PS domain and the legacy CS domain, and the MGCU has to create and manage a corresponding subscription instance for the user; when the MGCU receives an ISUP call containing voice message indication from the Voice Mail System in the legacy CS domain, MGCU has to convert the ISUP call into a NOTIFY message and send the NOTIFY message to the PS user for whom a subscription instance has been created.
Even SIP terminal users in the PS domain have to initiate subscriptions of voice message event packets through the SIP terminal interface, in order to use the Calling Tapping at Flash service.
It is seen from the above analysis, that in the implementation scheme of the Calling Tapping Flash service provided in ETSI Draft,
various possible cases are not considered; as the result, it is impossible to implement the scheme for voice message event packets based on SUBSCRIBE/NOTIFY mechanism if the user is located in the legacy CS domain while the voice mailbox is located in the PS domain;
the supplementary service control unit in PS domain and the MGCU are required to create and manage subscription instances of voice message event packets, resulting in increased memory overhead and reduced processing capacity;
SIP terminals (interfaces) are required to support subscription of voice message event packets; therefore, the implementation of the Calling Tapping at Flash service has to rely on the support of telephone feature and is difficult to generalize;
SIP users switched from legacy CS domain to PS domain are required to subscribe voice message event packets before they can use the Calling Tapping at Flash service, which brings inconvenience to user operations and uses; and
In order to keep the subscription instances of the voice message event packets valid permanently, both the SIP terminal and the supplementary service control unit have to update the subscription instances periodically by resending SUBSCRIBE messages, resulting in additional system load.